Players are free to shuffle their own decks as they choose. Once players have finished with their own decks, they have two things they must do with the opponent's deck.
The first action is to shuffle. Be careful not to bend their cards or destroy their sleeves in the process. After shuffling the opponent's deck, cut the deck, as shown below.
Then you may cut the opponent's deck. Carefully take the opponent's deck and take small stacks of cards from it, then put them back together in a different order. After that, the opponent must place their deck in the Deck Zone and not touch it again until they draw 5 cards to start a game.